Hardmasks and other photomasks are often utilized in lithography systems to manufacture the ICs, where successful production can require features on such photomasks have desired and uniform sizes. Accordingly, photomask manufacturers routinely evaluate feature sizing performance by measuring specific features in order to ensure that the photomasks include features that have the desired and uniform sizes. The features that are evaluated are generally referred to as critical dimensions (CDs), and are measured via optical systems and/or scanning electron microscopes (SEMs).
Optical metrology tools include reflectometers, ellipsometers, spectroscopic reflectometers, spectroscopic ellipsometers, polarized beam reflectometers, polarized beam spectroscopic reflectometers, scatterometers, spectroscopic scatterometers and optical CD measurement tools. Optical CD (OCD) measurement is useful because often only one measurement is required to analyze CDs, profiles, thicknesses, and sidewall angles without fracturing the wafer. However, as feature sizes have decreased below resolution limits of many OCD measurement tools, the use of SEMs has increased. Nonetheless, multiple optical and/or electron microscope instruments can be combined on a common platform to comprise a single metrology instrument that incorporates multiple spectroscopic metrology capabilities. In such arrangements, one or more processors may be utilized to analyze output signals generated by various detectors, processing the output signals individually or in combination to evaluate the characteristics of a sample.
Hardmasks formed by ash removable deposition (ARD) processing, particularly hardmasks comprising amorphous carbon, has recently gained popularity as a new approach for IC patterning. Amorphous carbon has a low etching rate, thus making its utilization beneficial when subsequent processing includes oxide or silicon dry etching. Additionally, amorphous carbon is easily removed by O2 plasma. Hence, patterning and stripping such hardmasks have little impact on profiles and CDs of features defined in underlying layers. Amorphous carbon also provides a high extinction coefficient k, which is beneficial during lithographic patterning. However, current optical measurement methods, such as ellipsometry and reflectometry, only extract the ordinary refractive index n and extinction coefficient k which are insufficient to accurately characterize amorphous carbon and other optically anisotropic materials.